**Description: **Extend wff definition to
include the membership connective between
classes.
(The purpose of introducing
here is to allow us to
express i.e. "prove" the wel 1449 of
predicate calculus in terms of the
wceq 1299 of set theory, so that we don't
"overload" the connective
with two syntax definitions. This is done to prevent ambiguity that
would complicate some Metamath parsers. The class variables and
are introduced
temporarily for the purpose of this definition but
otherwise not used in predicate calculus.) |